The Excel Developers Kit has been out of print for almost a decade, sadly there has been no update, but you can find a e-copy here:
http://msdn.microsoft.com/archive/default.asp?url=/archive/en-us/office97/html/edkfrnt.asp
Most of the stuff is out of date now and much of the C++ stuff just wont work - I understand that some of it didn’t work when it was published! I actually have the hard copy of this is book so if you really need any of the example files I might be able to get them for you?

The second is a link to some general C++ books, not read them myself but though I would post a link as they look good (and are free :-))
http://www.mindview.net/Books/TICPP/ThinkingInCPP2e.html

Enjoy!